Binder for batteries, and electrode compositions and batteries incorporating same

ABSTRACT

A binder for use in batteries which is in the form of an aqueous or nonaqueous dispersion or a powder comprising core-shell composite fine particles 0.05 to 1 μm in mean particle size and each having a core of a fibrillating polytetrafluoroethylene and a shell of a nonfibrillating polymer, the composite fine particles having a core-to-shell ratio by weight of 98:2 to 50:50, and electrode compositions and batteries incorporating the binder.

(TECHNICAL FIELD)

The present invention relates to a binder for batteries, and electrodecompositions and batteries incorporating the same.

(BACKGROUND ART)

In recent years, there is a growing demand for electric or electronicdevices which are compact and suitable to carry, such as audio taperecorders, video tape recorders with a built-in camera, personalcomputers and cellular phones. With this trend, compactness and highperformance are required of batteries, and various new batteries such asnickel-metal hydride batteries and lithium-ion batteries arecommercially available in addition to conventional lead batteries andnickel/cadmium batteries. The fabrication of batteries requires bindingvarious electrode materials including, for example, manganese dioxide(MnO₂), nickel hydroxide Ni(OH)₂ !, hydrogen storage alloy, lithiumcobalt dioxide (LiCoO₂), lithium nickel dioxide (LiNiO₂), lithiummanganese dioxide (LiMnO₂), carbon, graphite, etc. Suitable binders inwide use are fluorine-containing resin materials which have excellentchemical and thermal resistances and have binding properties.

For example, an aqueous dispersion of polytetrafluoroethylene (PTFE) isused in JP-A-236258/1988 for binding MnO₂, acetylene black, graphite,etc. serving as positive electrode materials for lithium primarybatteries. JP-B-10980/1994 gives an example of binding a manganeseoxide, and carbon black or activated carbon with an aqueous dispersionof PTFE for an air-zinc battery. On the other hand, also known ispolyvinylidene fluoride (PVDF) used as an example of binder. In regardto nickel-metal hydride batteries, JP-A-44964/1994 describes anelectrode prepared by mixing a hydrogen storage alloy, carbonyl nickelpowder or like conductive agent with a solution of PVDF to make themixture into a sheet. JP-A-249860/1992 also discloses the use of PVDF asan example of binder for lithium ion secondary batteries, i.e., the useof an N-methylpyrrolidone solution of PVDF for a positive electrodecomprising LiCoO₂ or like lithium-containing oxide and graphite and fora negative electrode of carbonaceous material in preparing therespective mixtures and making each mixture into a sheet.

PTFE has the property of being liable to fibrillate by compressiveshearing force, and therefore readily produces fibrils when mixed withother powder material and acts to interlock the particles. However, auniform mixture is difficult to prepare because of excessivefibrillation, and PTFE is likely to impair the characteristics ofelectrodes if added in an amount more than is necessary. To obtain auniform mixture, PTFE is usually frequently used in the form of acolloidal aqueous dispersion rather than a powder. The dispersionnevertheless has the problem of fibrillating excessively andencountering difficulty in forming a uniform mixture. Further the use ofthe aqueous dispersion eventually requires the step of removing a largeamount of water and the surfactant contained therein as a stabilizer byheating, and the surfactant and water are likely to adversely affect thecharacteristics of some types of batteries. For example, thelithium-containing oxides for use in the positive electrode of thelithium ion secondary battery include LiNiO₂ which is especiallyreactive with water and is not usable with the aqueous dispersion ofPTFE. In the case where PVDF is used as a binder, the resin is solublein organic solvents and can therefore be mixed in the form of a solutionwith electrode materials, consequently forming a uniform mixture andobviating the need for the step of removing water or surfactant unlikethe PTFE aqueous dispersion. However, the property of being soluble inorganic solvents renders PVDF prone to swell in organic electrolytes,such as propylene carbonate, dimethoxyethane and γ-butyrolactone, foruse as battery materials, and swelling is likely to impair theperformance of batteries. PVDF has another drawback in being inferior toPTFE in binding properties.

An object of the present invention is to provide a binder for batterieswhich can be mixed with electrode materials more uniformly than the PTFEaqueous dispersion, is usable also for electrode materials susceptibleto an adverse influence of water, does not swell in organic electrolytesunlike PVDF and has excellent binding properties, and electrodecompositions and batteries incorporating the binder.

(DISCLOSURE OF THE INVENTION)

The present invention provides a binder for batteries in the form of anaqueous or nonaqueous dispersion or a powder and comprising core-shellcomposite fine particles 0.05 to 1 μm in mean particle size and eachhaving a core of a fibrillating polytetrafluoroethylene and a shell of anonfibrillating polymer, the composite fine particles having acore-to-shell ratio by weight of 98:2 to 50:50, and electrodecompositions and batteries incorporating the binder.

The fibrillating PTFE forming the cores of composite fine particles ofthe invention is similar to that of fine particles having a meanparticle size of 0.05 to 1 μm and prepared from knowntetrafluoroethylene (TFE) by emulsion polymerization. The PTFE is thesame material as commercial PTFE fine powder obtained by coagulating anddrying an emulsion polymer, or as fine particles constituting a PTFEaqueous dispersion prepared from the emulsion polymer by concentrationand stabilization. More specific production processes are made known byJP-B-4643/1962, JP-B-14466/1971, JP-B-26242/1981, etc. The fibrillationis a usual property of a PTFE having such a high molecular weight (atleast 10⁸ poises in melt viscosity at 380° C.), which is notmelt-processable. Such a PTFE is up to 2.210, preferably 2.200 to 2.130,in terms of standard specific gravity (ASTM D-1457). (The smaller thestandard specific gravity, the higher is the molecular weight.) The meltviscosity is determined with use of a Koka flow tester, manufactured byShimadzu, Ltd., by filling the polymer powder into the cylinder, 11.3 mmin inside diameter, holding the powder at a temperature of 380° C. for 5minutes, thereafter extruding the melt through an orifice, 0.21 cm ininside diameter (2R) and 0.8 cm in length (L), with a load (7 or 32 kg)applied to the piston, and measuring the rate of outflow (Q: cm² /sec),followed by calculation from the equation given below. ##EQU1##

If the standard specific gravity is greater than 2.210, that is, if themolecular weight is lower, the polymer is less likely to fibrillate.When the PTFE has a high molecular weight whose standard specificgravity is smaller than 2.130, this type of polymer is difficult toproduce and is not suitable actually although still retaining thefibrillating property inherent therein. In the emulsion polymerizationof TFE, a small amount (0.001 to 2 wt. %) of other fluorine-containingmonomer (such as chlorotrifluoroethylene, hexafluoropropene,fluoroalkylethylene or fluoroalkyl fluorovinyl ether) may becopolymerized therewith as the case may be. Fine particles of such aso-called modified PTFE retain the fibrillating property intact and areincluded in the invention.

Although the standard specific gravity or molecular weight serves as anindex of the fibrillating property, whether or not the fine particlesprepared by emulsion polymerization are extrudable in the form of apaste, provides an actual standard in the case of the present invention.It is desired that a continuous extrudate be available and that theelongation thereof be at least about 10% at room temperature.

The nonfibrillating polymer for forming the shells of the composite fineparticles is a low-molecular-weight polytetrafluoroethylene,polyvinylidene fluoride, fluorine-containing copolymer comprising atleast one of tetrafluoroethylene, vinylidene fluoride (VDF) andchlorotrifluoroethylene (CTFE) as a component monomer, or at least onepolymer selected from among those prepared by polymerizing a hydrocarbonmonomer which is liquid at room temperature. The low-molecular-weightPTFE is preferably less than 10⁸ poises, more preferably 10² to 10⁷poises, in melt viscosity at 380° C.

Examples of fluorine-containing copolymers comprising at least one ofTFE, VDF and CTFE as a component monomer are a copolymer (generallyknown as FEP) of TFE and hexafluoropropylene (HFP), copolymer (generallyknown as PFA) of TFE and perfluoro(alkyl vinyl ether) (PFAVE),terpolymer of TFE, HFP and PFAVE, copolymer of TFE and aperfluoro(alkyl)ethylene, copolymer (generally known as ETFE) consistingessentially of ethylene and TFE, copolymer (generally known as ECTFE)consisting essentially of ethylene and CTFE, copolymer of TFE and CTFE,co- or ter-polymer of VDF and TFE, and/or HFP, terpolymer of VDF, TFEand CTFE, etc. Such copolymers will exhibit the properties of resinshaving a glass transition temperature higher than room temperature orthe properties of rubbers whose glass transition temperature is lowerthan room temperature, depending on their composition, whereas usefulcopolymers are not limited specifically as to such properties, nor arethey limited specifically in molecular weight. In the case of thecopolymers of TFE, however, it is desired that more than 2 wt. % of theother monomer(s) be present.

Examples of hydrocarbon monomers which are liquid at room temperatureare methyl acrylate, methyl methacrylate, ethyl acrylate, ethylmethacrylate, n-propyl acrylate, n-propyl methacrylate, isopropylacrylate, isopropyl methacrylate, lauryl acrylate, stearyl acrylate andlike α,β-ethylenically unsaturated carboxylic acid esters,2-hydroxyethyl acrylate, 2-hydroxyethyl methacrylate, 3-hydroxypropylmethacrylate and like hydroxyalkyl esters of α,β-ethylenicallyunsaturated carboxylic acids, diethylene glycol methacrylate and likealkoxyalkyl esters of α,β-ethylenically unsaturated carboxylic acids,acrylamide, methylol methacrylamide and like α,β-ethylenicallyunsaturated carboxylic acid amides, acrylic acid, methacrylic acid,itaconic acid, maleic anhydride, maleic acid, fumaric acid, crotonicacid and like α,β-ethylenically unsaturated carboxylic acids, styrene,alkylstyrenes, acrylonitrile, vinylpyrrolidone, alkyl vinyl ethers,pyrrole, etc. Homopolymers or copolymers of such monomers are usable asthe polymer for the shells. The α,β-ethylenically unsaturated carboxylicacids, hydroxyalkyl-containing α,β-ethylenically unsaturated carboxylicacid esters and amides are excessively hydrophilic and encounterdifficulty in forming shells when in the form of homopolymers, so thatit is desired to copolymerize these monomers with other hydrophobicmonomer.

Among the nonfibrillating polymers given above for forming the shells,preferable are the polymers containing TFE since a shell formingreaction can be conducted easily, continuously following the preparationof the fibrillating PTFE for the cores.

The core-shell composite fine particles of the present invention can beprepared according to the process disclosed in JP-A-154842/1992 when alow-molecular-weight PTFE is used for the shells, the process ofWO94/1475 when a VDF-type resin is used, the process of JP-B-63584/1988when a copolymer of TFE and CTFE is used for the shells, the process ofJP-A-158651/1990 when ETFE or ECTFE is used for the shells, or theprocess of JP-A-312836/1988 when a polymer of α,β-unsaturated carboxylicacid ester is used for the shells. While the particles are 0.05 to 1 μmin mean size, particles greater than 1 μm can be obtained as byso-called seed polymerization. However, dispersions containing fineparticles of large size are unstable to preserve, hence a problem. Toosmall particles are less likely to undergo sufficient fibrillation whenmixed with electrode materials.

The composite fine particles of the invention have a core-to-shell ratioby weight of 98:2 to 50:50. If the proportion of shells is too small,the particles will not be mixed with electrode materials uniformly orare not dispersable smoothly in organic media. An excessive proportionof shells impairs the inherent binding properties of the particles, oris more likely to entail the problem of swelling within batteries in thecase where the shells are formed of a VDF-type polymer or a polymercomprising a hydrocarbon monomer which is liquid at room temperature.Preferably, the ratio is 95:5 to 60:40.

The binder of the invention for use in batteries is in the form of anyof an aqueous dispersion, nonaqueous dispersion or powder. Being in theform of composite fine particles proves most effective when the binderis a nonaqueous dispersion or powder although the form of the binder isselected according to the type of battery and the kind of electrodematerials. The PTFE conventionally used as a binder is a fine powder oraqueous dispersion, but no nonaqueous dispersion thereof is available.The nonaqueous dispersion is easily mixed with electrode materials, doesnot contain a large amount of surfactant unlike the aqueous dispersion,and accordingly does not require eventual removal thereof. Further inthe case of nonaqueous electrolyte secondary batteries wherein alithium-containing oxide is used for the positive electrode, the absenceof water is preferred, and the nonaqueous dispersion is thereforedesirable. On the other hand, the powder of the invention, which can bemixed as it is with electrode materials effectively unlike the finepowder, is redispersable in aqueous media containing a surfactant or inorganic media. This affords greater freedom in selecting a particularmethod of mixing with electrode materials. Thus, a wide variety ofmixing methods are usable for the battery binder of the inventionaccording to the kind of electrode materials.

It is suitable that the concentration of polymer fine particles in theaqueous dispersion or nonaqueous dispersion be 5 to 65 wt. %. Lowerconcentrations lead to a poor efficiency when the dispersion is mixedwith electrode materials, while excessively high concentrations renderthe dispersion highly viscous and result in the drawback that thedispersion is difficult to handle. If the aqueous dispersion istransported or handled as it is, a problem is liable to arise in thestability of the dispersion, therefore it is desirable to adjust the pHto an alkalinity value of 7 to 11 and add a surfactant to the dispersionfor stabilization. The type of surfactant is not limited specifically.At least one of anionic, cationic and nonionic types is usable. Nonionicsurfactants are generally desirable. Examples are a group of surfactantsof the fatty acid ester of polyoxy-compound such as glycol esters offatty acids, and sorbitol and mannitol esters of fatty acids, and agroup of surfactants of the polethylene oxide condensate type such aspolyethylene oxide condensates of higher fatty acids, higher alcohols,higher alkylamines, higher fatty acid amides, higher alkylmercaptans andalkylphenols. The surfactant is used in an amount of 1 to 20% based onthe total weight of the fine particles in the aqueous dispersion. Toosmall an amount leads to insufficient dispersion stability, whereasexcessive amounts adversely affect the performance of batteries. Thesurfactant is not essential to the battery and can therefore bedispensed with.

The powder of the present invention is prepared from an aqueousdispersion obtained by emulsion polymerization, by separating polymerfine particles from the dispersion by a usual method of coagulation(see, for example, the specification of U.S. Pat. No. 2,593,583),followed by drying. The drying must be effected at a low temperature soas not to permit the fusion of the shells of the fine particles. Themean particle size of the powder is suitably 1 to 30 μm although notlimited specifically. A powder smaller than 1 μm in particle size is notavailable substantially. Particles greater than 30 μm in size can not bemixed with electrode materials effectively. The particle size iscontrollable by varying the concentration or temperature of the aqueousdispersion and the intensity of stirring during the coagulation. Thepowder is in the form of agglomerates of 0.05- to 1-μm fine particles,and the mean particle size is the secondary size thereof. On the otherhand, the nonaqueous dispersion of the invention can be readily preparedby redispersing the powder in a dispersant solvent mechanically orultrasonically. Alternatively, the aqueous dispersion can be made into anonaqueous dispersion by the phase transfer process described inJP-B-17016/1974 without resorting to the powder forming process.However, the polymer fine particles in the nonaqueous dispersion areless likely to be finely dispersed therein unlike those in the aqueousdispersion, and become agglomerated to some extent. The agglomerates ofparticles are usually 1 to 3 μm in size. Nevertheless, the fineparticles are not coherent but are merely agglomerate. The term "meanparticle size" as defined in claim 1 of the invention does not mean thesize of the agglomerates but refers to the size of the core-shellcomposite fine particles substantially having a binding effect.

The dispersant for the nonaqueous dispersion is not limited particularlyinsofar as it is an organic liquid capable of wetting the fine particlesof TFE-type polymer. Examples of dispersants preferred for use arearomatic hydrocarbons such as benzene, toluene and xylene, hydrocarbonhalides such as carbon tetrachloride and trichloroethylene, ketones andesters such as methyl isobutyl ketone, diisobutyl ketone and butylacetate, alcohols such as water-soluble methanol, ethanol and isopropylalcohol, N-methylpyrrolidone, etc. The organic liquid, such as carbontetrachloride, trichloroethylene or diisobutyl ketone, which is lesslikely to wet the particles when used singly, can be made into anorganosol by adding a small amount of an oil-soluble surfactant. In viewof the dispersability of the fine particles, handling-ability thereof inpreparing the electrodes, or toxicity, especially desirable organicliquids for use are isopropyl alcohol, methyl isobutyl ketone andN-methylpyrrolidone. The use of the organosol for preparing electrodematerials entails the advantage that the resulting dispersion is easierto dry than the aqueous dispersion and less likely to permit thesurfactant to adversely affect the electrode than the aqueousdispersion.

The electrode composition of the present invention comprises a powderyelectrode material and 0.1 to 10 wt. % of core-shell composite fineparticles 0.05 to 1 μm in mean particle size and each having a core ofthe fibrillating polytetrafluoroethylene and a shell of anonfibrillating polymer. Examples of powdery electrode materials areactive materials for batteries, conductive agents, catalysts, etc.although they vary with the type of batteries. The active materials forbatteries are divided generally into positive electrode active materialsand negative electrode active materials. Examples of positive electrodeactive materials are powders of lithium cobalt dioxide, lithium nickeldioxide, lithium manganese oxide compounds, lithium vanadium oxidecompounds, lithium iron oxide, lithium compounds, i.e., complex oxidesof these compounds and transition metal oxides, manganese dioxide, zincoxide, nickel oxide, nickel hydroxide, copper oxide, molybdenum oxide,carbon fluoride, etc. Such examples also include N-fluoropyridiniumcompounds, N-fluorosulfonamide compounds, N-fluoroquinuclidiniumcompounds, N-fluoro-1,4-diazoniabicyclo 2,2,2!octane compounds,N-fluorodisulfonimide compounds, N-fluoroamide compounds,N-fluorocarbamate compounds, N-fluoropyridone compounds (seeJP-A-6756/1995) and further mixtures of salts of these compounds andlithium salts, especially a mixture of N-fluoropyridinium salt andlithium salt in a lithium salt-to-N-fluoropyridinium salt molar ratio inthe range of 0.1 to 5. On the other hand, examples of negative electrodeactive materials are metallic lithium and like alkali metals, alloysthereof, alkali metal absorbing carbon materials, zinc, cadmiumhydroxide, hydrogen storage alloys, etc. Examples of conductive agentsare powders of active carbon, carbon black, acetylene black, graphite,conductive polymers typical of which is polyaniline, etc. The catalystis, for example, finely divided platinum for use in fuel cells and thelike. The electrode composition of the invention may have incorporatedtherein hydrophilic binders such as polyvinyl alcohol, polyacrylic acid,polyacrylic acid salts, methyl cellulose, carboxymethyl cellulose,hydroxypropyl cellulose and polyacrylamide, organic electrolytescomprising one or a mixture of propylene carbonate, ethylene carbonate,dimethoxyethane and γ-butyrolactone which have dissolved therein lithiumperchlorate, lithium boron fluoride, cesium carbonate or the like, andinorganic electrolytes such as aqueous solution of potassium hydroxide,in addition to the above electrode materials. The binder of theinvention is suitable for use in batteries wherein a nonaqueouselectrolyte is used, especially for use in nonaqueous electrolytesecondary batteries wherein a lithium compound is used for the positiveelectrode.

(BRIEF DESCRIPTION OF THE DRAWINGS)

FIG. 1 is a photo of scanning electron microscope showing the structureof a binder powder obtained in Reference Example 1.

FIG. 2 is a photo of scanning electron microscope at a magnification of×10,000 showing the configuration of fibers in an electrode obtained inExample 1.

FIG. 3 is a diagram showing a battery of simplified construction forevaluating charge-discharge characteristics with use of the positiveelectrodes prepared in Examples 3 and 4 and Comparative Example 3.

(BEST MODE OF CARRYING OUT THE INVENTION)

The present invention will be described below in greater detail withreference to reference examples, examples and comparative examples.

In the present invention, paste extrusion tests were conducted in thefollowing manner.

An aqueous dispersion is heated at 100° C. to evaporate off the water,and the emulsifier used for polymerization and the surfactant, added forstabilization after concentration as the case may be, are then fullyextracted with acetone. With 50 g of a cakelike polymer obtained isthereafter mixed in a glass bottle 10.8 g of an extrusion aid (brandname "IP1620," product of Idemitsu Petrochemical Co., Ltd.) which is ahydrocarbon oil, followed by standing at room temperature (25°±2° C.)for 8 hours to make the polymer fully compatible with the extrusion aid.Subsequently, the mixture is filled into an extrusion die equipped witha cylinder (25.4 mm in inside diameter) and having at its lower end anorifice, 30 deg in reduction angle, 2.54 mm in inside diameter and 7 mmin land length, and is held for 1 minute with a load of 60 kg applied toa piston inserted in the cylinder. Immediately thereafter, the mixtureis extruded indoors at a ram velocity (velocity of depression of thepiston) of 20 mm/min. When a continuous extrudate is available, theassisting agent is dried at 80° C., and the extrudate is subjected to atensile test at room temperature along the direction of extrusion at arate of pulling of 20 mm/min.

REFERENCE EXAMPLE 1

An aqueous dispersion of core-shell composite fine particles with coresof PTFE and shells of ETFE (core-to-shell weight ratio 91:9) wasprepared according to JP-A-158651/1990, Example 1.

First, into a 3-liter autoclave of stainless steel (SUS 316) equippedwith anchor agitator blades of stainless steel and atemperature-adjusting jacket were placed 1450 ml of deionized water, 90g of liquid paraffin and 1.5 g of ammonium perfluorooctanate. The airwithin the system was replaced by nitrogen gas three times and bytetrafluoroethylene (TFE) twice to remove oxygen, the internal pressurewas thereafter adjusted to 1.0 MPa with TFE, and the mixture wasmaintained at 70° C. with stirring at 280 r.p.m.

Next, 0.2 g of hexafluoropropene (HFP) and 50 ml of an aqueous solutionhaving 80 mg of ammonium persulfate (APS) dissolved therein as apolymerization initiator were placed into the system to start areaction. During the reaction, TFE was continuously supplied so as tomaintain the internal pressure at 1.0 Mpa at all times while maintainingthe internal temperature at 70° C. with stirring at 280 r.p.m.

The stirring and supply of TFE were discontinued when the amount of TFEconsumed by the reaction after the addition of the initiator reached 390g, and the gas was released until the internal pressure of the autoclavedropped to 0.3 MPa. A gas mixture of ethylene and TFE containing 48 mol% of ethylene was further supplied to the autoclave through another lineto a pressure of 1.0 MPa, and a continued reaction was effected withstirring resumed and the pressure maintained. Upon consumption of 37 gof the gas mixture, the stirring was discontinued, and the gas wasvented to terminate the reaction.

An aqueous dispersion was obtained which was found to contain a solidcomponent at a concentration of 22.0 wt. % (approximately equal to theconcentration of the resulting polymer), as determined by evaporating aportion of the dispersion to dryness. The polymer fine particles had amean particle size of 0.20 μm as measured using a photo of electronmicroscope.

To a portion of the aqueous dispersion was added a commercial nonionicsurfactant, i.e., Triton X-100 (product of Rohm & Haas Co.), in anamount of 5.0 wt. % based on the weight of the polymer present. Themixture was then adjusted to a pH of 9.0 with aqueous ammonia, followedby evaporation of water in a vacuum to obtain a concentrate having apolymer solid content of 60 wt. %. The polymer fine particles in theconcentrated aqueous dispersion had the same mean particle size of 0.20μm as the original aqueous dispersion, as measured with use of a photoof electron microscope.

Two liters of the aqueous dispersion resulting from the reaction wasplaced into a 5-liter container of stainless steel equipped with aturbine impeller mixer, and stirred at room temperature with addition of10 g of ammonium carbonate to effect coagulation. The wet particlesobtained were dried in an electric oven at 120° C. for 16 hours toobtain a powder. When checked for mean particle size by the dry laserbeam scattering method (particle size distribution measuring device ofthe laser diffraction type, HELOS & RODOS, produced by Shinpatech Co.,Ltd.), the powder was 5 μm in mean particle size. FIG. 1 is a photo ofscanning electron microscope showing the structure of the powder. Thephotograph reveals that the powder comprises agglomerates of 0.20-μmfine particles. When tested for paste extrusion, the aqueous dispersiongave no continuous extrudate.

Subsequently, 10 g of the dry powder and 90 g of isopropyl alcohol wereplaced into a 200-ml container and irradiated with ultrasonic waves at afrequency of 20 kHz and output of 100 W to prepare a nonaqueousdispersion. The fine particles dispersed in the isopropyl alcohol werechecked for mean particle size by an automatic particle sizedistribution measuring device (CAPA-700), product of Horiba SeisakushoCo., Ltd., in a spontaneous settling mode to find that the size was 1.25μm. Although this value is greater than the mean particle size of theoriginal fine particles since the composite fine particles in theisopropyl alcohol agglomerate to some extent, the photo of scanningelectron microscope of the polymer revealed that the fine particlesforming the agglomerates as basic units had the same mean particle sizeas the fine particles in the original aqueous dispersion as prepared bypolymerization.

REFERENCE EXAMPLE 2

A polymer was prepared by the reaction of Reference Example 1 accordingto JP-A-158651/1990 without effecting the shell forming reaction. Thefine particles of the polymer were found to be 0.19 μm in mean particlesize by measurement with a photo of electron microscope. A portion ofthe aqueous dispersion obtained was treated in the same manner as inReference Example 1 for coagulation and drying to obtain an agglomeratepowder, which was 400 μm in mean particle size and 2.180 in standardspecific gravity, hence a high molecular weight like the commercial PTFEfine powder. The paste extrusion test of the polymer from the aqueousdispersion afforded a continuous extrudate, which was found to be 300%in elongation by a tensile test.

When the powder was dispersed in isopropyl alcohol as in ReferenceExample 1, the dispersed particles were not smaller than the originalparticle size. The remaining portion of the aqueous dispersion wasconcentrated by the same procedure as in Reference Example 1.

REFERENCE EXAMPLE 3

An aqueous dispersion of PTFE was prepared by the same procedure as inReference Example 1 with the exception of using a 6-liter autoclave ofstainless steel, 2950 ml of deionized water, 120 g of liquid paraffin,3.0 g of ammonium perfluorooctanate, 0.4 g of hexafluoropropene and 12mg of APS as a polymerization initiator, and discontinuing stirring andsupply of TFE and releasing the gas when the amount of TFE consumed bythe reaction after the addition of the initiator reached 330 g. Thedispersion contained a solid component at a concentration of 24 wt. %,and the polymer fine particles were 0.22 μm in mean particle size.

Next, 401.6 g of the aqueous dispersion and 153.4 g of deionized waterwere placed into a 1-liter separable flask and mixed together withaddition of 5 g of an aqueous solution containing 0.5 g of ammoniumperfluorooctanate dissolved therein, followed by stirring at 150 r.p.m.

The interior of the flask was then heated to 70° C. in a nitrogenstream, and a solution of 100 mg of the polymerization initiator, APS,in 19.9 g of water was further placed in. The mixture was reacted whileadding 3.4 g of methyl methacrylate monomer (MMA) in six dividedportions at an interval of 10 minutes. The stirring was discontinuedafter 90 minutes, and the mixture was withdrawn from the system.

An aqueous dispersion was obtained which contained a solid component ata concentration of 20 wt. % as determined by evaporating a portion ofthe dispersion to dryness. The polymer fine particles were 0.25 μm inmean particle size. The amount of PMMA of the shells was 16.2 wt. % asdetermined by thermal decomposition. A 250 g quantity of the aqueousdispersion was dried in a vacuum dryer at 50° C. for 24 hours, givingabout 58 g of a white solid. A powder with a mean particle size of about6 μm was obtained by pulverizing the solid.

EXAMPLE 1

A 2.5 g quantity of the nonaqueous dispersion prepared in ReferenceExample 1, 20 g of manganese dioxide and 2.5 g of conductive carbon wereplaced into an automatic mortar and mixed together for 15 minutes. Themixture made into a paste was thereafter applied to an 80-mesh metal netplated with nickel and dried at 100° C. Subsequently, an electrode wasprepared by rolling the coated net with rolls. The paste was made fromthe mixture and the electrode material remained fixed to the metal netbecause the composite fine particles underwent fibrillation. FIG. 2 is aphoto of scanning electron microscope showing the configuration offibers in the electrode at a magnification of ×10,000. The photographreveals fibrils produced.

EXAMPLE 2

A 2.8 g quantity of the concentrate of the aqueous dispersion obtainedin Reference Example 1, 20 g of manganese dioxide and 2.5 g ofconductive carbon were placed into an automatic mortar along with 20 mlof water, followed by mixing for 30 minutes. The mixture made into apaste was thereafter applied to an 80-mesh metal net plated with nickeland dried at 100° C. Subsequently, an electrode was prepared by rollingthe coated net with rolls. A photo of scanning electron microscope ofthe electrode was similar to that of FIG. 2.

COMPARATIVE EXAMPLE 1

While the powder of Reference Example 2 failed to finely disperse inisopropyl alcohol, 20 g of manganese dioxide and 2.5 g of conductivecarbon were added to a mixture of 2.5 g of the powder and 22.5 g ofisopropyl alcohol, followed by mixing in an automatic mortar as inExample 1. However, the mixture failed to become a paste in its entiretywith fine particles forming a mass.

COMPARATIVE EXAMPLE 2

The procedure of Example 2 was repeated using 2.8 g of the concentrateof the aqueous dispersion obtained in Reference Example 2, affording apaste and an electrode. A photo of scanning electron microscope revealedthat the state of fine particles as dispersed was inferior to that ofExample 2.

EXAMPLES 3 AND 4, AND COMPARATIVE EXAMPLE 3

One hundred parts by weight of lithium nickel dioxide having a meanparticle size of 5 μm (product of Honjo Chemical Co., Ltd.), 3 parts byweight of acetylene black (product of Denki Kagaku Kogyo K.K.) as aconductive agent and 5 parts by weight of the powder of ReferenceExample 1 as a binder were mixed together, and NMP was further added sothat the resulting mixture would be 50% in solids concentration,followed by treatment in a mixer for 20 minutes to prepare a paste.

The paste was applied to 25-μm-thick aluminum foil and dried at 100° C.The coating as dried had a thickness of 120 μm. A positive electrodesheet 1 was prepared by rolling the resulting sheet with rolls until thethickness of the coating reduced to 100 μm. The positive electrodesheets 3, 2 were similarly prepared using the powders of ReferenceExamples 3, 2, respectively. A uniform coating was not available for thepositive electrode sheet 2 with white spots produced on its surface.Subsequently, 100 squares, 1 mm×1 mm, were formed on each sheet bycutting the coating crosswise with a cutter knife, and the number ofsquares remaining without being peeled off was counted. Table 1 showsthe result.

The positive electrode sheets were further tested for the evaluation ofbattery characteristics in the following manner. A rectangular piece,3×5 cm, was cut out from the sheet, and a lead attached to the piece toprepare a positive electrode. Metallic lithium foil was used as anegative electrode. A secondary battery as shown in FIG. 3 wasfabricated by positioning the electrodes as opposed to each other with apolypropylene separator interposed therebetween and using a solution of1 mol/liter of lithium perchlorate in propylene carbonate as anelectrolyte. The battery was repeatedly charged and discharged under theconditions of charge-discharge current density 1 mA/cm², chargetermination voltage 4.3 V and discharge termination voltage 3.0 V todetermine the time when the capacity decreased below 60% of the initialcapacity as cycle life. Table 1 shows the average result obtained bysubjecting 10 batteries to the cycle test for each positive electrodesheet. The table reveals that the positive electrode sheets 1, 3 areexceedingly superior to the positive electrode sheet 2 in adhesion andcycle life.

                  TABLE 1                                                         ______________________________________                                                       Positive  Positive  Positive                                                  electrode electrode electrode                                                 sheet 1   sheet 3   sheet 2                                    ______________________________________                                        Number of remaining squares                                                                   90        91       20                                         Cycle life (number of cycles)                                                                250       290       32                                         ______________________________________                                    

(INDUSTRIAL APPLICABILITY)

The invention provides a binder for use in batteries which can be mixedwith electrode materials more uniformly than the conventional PTFEaqueous dispersion, is usable for electrode materials susceptible to anadverse influence of water, does not swell in organic electrolytesunlike PVDF and has excellent binding properties, and electrodecompositions and batteries having the binder incorporated therein.

We claim:
 1. A composition for an electrode comprising 0.1 to 10 wt. %of core-shell composite fine particles and, as the balance, a powderyelectrode material, the composite fine particles being 0.5 to 1 μm inmean particle size, each comprising a core of a fibrillatingpolytetrafluoroethylene and a shell of a nonfibrillating polymer, andhaving a core-to-shell ratio by weight of 98:2 to 50:50.
 2. A nonaqueouselectrolyte secondary battery comprising a positive electrode, saidpositive electrode comprising a lithium compound as an active material,an electrically conductive agent and a binder; a negative electrode anda nonaqueous electrolyte, wherein the binder comprises core-shellcomposite fine particles 0.05 to 1 μm in mean particle size and eachhaving a core of a fibrillating polytetrafluoroethylene and a shell of anonfibrillating polymer, the composite fine particles having acore-to-shell ratio by weight of 98:2 to 50:50.
 3. The electrodecomposition of claim 1, wherein said nonfibrillating polymer is selectedfrom the group consisting of a low molecular weightpolytetrafluoroethylene, polyvinylidene fluoride, a copolymer oftetrafluoroethylene, a copolymer of vinylidene fluoride, a copolymer ofchlorotrifluoroethylene and a polymer of a hydrocarbon monomer that isliquid at room temperature.
 4. The electrode composition of claim 1,wherein said nonfibrillating polymer is a polymer comprisingtetrafluoroethylene.
 5. The electrode composition of claim 1, whereinsaid nonfibrillating polymer is a copolymer of tetrafluoroethylene and amonomer selected from the group consisting of hexafluoropropylene, aperfluoro(alkylvinyl ether), a perfluoro(alkyl)ethylene, ethylene,vinylidene fluoride and chlorotrifluoroethylene; a copolymer ofchlorotrifluoroethylene and ethylene; a copolymer of vinylidene fluorideand hexafluoropropylene; a terpolymer of tetrafluoroethylene,hexafluoropropylene and perfluoro(alkyl vinyl ether); a terpolymer oftetrafluoroethylene, vinylidene fluoride and hexafluoropropylene and aterpolymer of tetrafluoroethylene, vinylidene fluoride andchlorotrifluoroethylene.
 6. The electrode composition of claim 1,wherein the composite fine particles have a core-to-shell ratio byweight of 95:5 to 60:40.
 7. The electrode composition of claim 1,wherein said powdery electrode material comprises a positive electrodeactive material.
 8. The electrode composition of claim 1, wherein saidpositive electrode active material is a lithium compound.
 9. Theelectrode composition of claim 1, wherein said powdery electrodematerial comprises a negative electrode active material.
 10. Thenonaqueous electrolyte secondary battery of claim 2, wherein saidnonfibrillating polymer is selected from the group consisting of a lowmolecular weight polytetrafluoroethylene, polyvinylidene fluoride, acopolymer of tetrafluoroethylene, a copolymer of vinylidene fluoride, acopolymer of chlorotrifluoroethylene and a polymer of a hydrocarbonmonomer that is liquid at room temperature.
 11. The nonaqueouselectrolyte secondary battery of claim 2, wherein said nonfibrillatingpolymer is a polymer comprising tetrafluoroethylene.
 12. The nonaqueouselectrolyte secondary battery of claim 2, wherein said nonfibrillatingpolymer is a copolymer of tetrafluoroethylene and a monomer selectedfrom the group consisting of hexafluoropropylene, a perfluoro(alkylvinylether), a perfluoro(alkyl)ethylene, ethylene, vinylidene fluoride andchlorotrifluoroethylene; a copolymer of chlorotrifluoroethylene andethylene; a copolymer of vinylidene fluoride and hexafluoropropylene; aterpolymer of tetrafluoroethylene, hexafluoropropylene andperfluoro(alkyl vinyl ether); a terpolymer of tetrafluoroethylene,vinylidene fluoride and hexafluoropropylene and a terpolymer oftetrafluoroethylene, vinylidene fluoride and chlorotrifluoroethylene.13. The nonaqueous electrolyte secondary battery of claim 2, wherein thecomposite fine particles have a core-to-shell ratio by weight of 95:5 to60:40.
 14. The electrode composition of claim 1, wherein saidcomposition is prepared from a mixture of said electrode material and anonaqueous dispersion or a powder comprising said core-shell compositefine particles, said powder having a size of 1 to 30 μm.
 15. Thenonaqueous electrolyte secondary battery of claim 2, wherein saidpositive electrode is prepared from a mixture of a lithium compound, anelectrically conductive agent and a nonaqueous dispersion or a powdercomprising said core-shell composite fine particles, said powder havinga particle size of 1 to 30 μm.